1. Field of the Invention
The present invention relates to a process for improving the cetane number or index of viscoreduced diesel fuels and to such diesel fuels, per se, having improved cetane numbers.
2. Description of the Prior Art
Diesel fuels proper are median distillates, intermediate between light distillates, gas and gasolines and heavier products, produced by distillation at atmospheric pressure, at which the refining of crude oils begins. Diesel fuels are essentially intended for use as fuels for diesel engines and for domestic heating.
The diesel fuels, which are thus derived directly from crude oils, or direct diesel oils, must have a sufficiently high compression ignition rating, expressed by their cetane number, which is comparable to the octane number rating for gasoline.
The desirability of having available to industry such diesel fuels having a high cetane number has been recently described, for example in the European patent published under No. 0,252,606. Presently, cetane numbers higher than about 45, preferably higher than 50, are considered desirable.
One proposed solution to ensure a satisfactory cetane number for direct diesel fuels entails incorporating an additive therein. The disadvantage of such a technique is that the selection of the additive is restricted to one that is simultaneously relatively insoluble in water and chemically stable relative thereto, sufficiently soluble in the diesel fuel under the preferred conditions of use, stable in diesel fuel at all storage temperatures, and finally, noncorrosive and nontoxic.
The distillation in a vacuum of products heavier than diesel fuel, or atmospheric residue, in turn yields a heavy residue or vacuum residue, which is then subjected to a viscosity reducing operation.
This operation, also designated "visbreaking", is a thermal cracking process, which reduces, at its name implies, the viscosity of the petroleum product to which it is applied. It is essentially different in its nature, for example from catalytic cracking, to which such petroleum product may also be subjected. It yields a product, designated a viscoreduction diesel fuel, which itself is essentially different from a direct diesel fuel or the diesel fuel emanating from catalytic cracking.
Viscoreduced diesel fuels cannot be used as such either as motor fuels or for domestic heating, in view of their excessively low cetane number, which is not sufficiently improved by the additives suitable for the direct diesel fuels.
For a definition of the terms employed herein, such as motor diesel fuel, cetane number, etc., reference may be made to the following texts: Le langage petrolier (The language of petroleum), Gauthier-Villars, Paris (1964); Le Petrole, raffinage et genie chimique (Petroleum, refining and chemical engineering), edited by Pierre Wuithier, 2nd edition, Technip Publisher, pp. 16, 17, 29, 30 (1972), or the article by D. Indritz, "Preprints, Symposium of the Chemistry of Cetane Number Improvement", Miami Beach, Apr. 28-May 3, 1985, pp. 282-286.